Docstring extraction functionality from within Docutils is still under
development. There is most of a source code parsing module in
docutils/readers/python/moduleparser.py. I (David Goodger) haven't
worked on it in a while, but I do plan to finish it eventually. Ian
Bicking wrote an initial front end for my moduleparser module, in
sandbox/ianb/extractor/extractor.py. There has also been some recent
activity in sandbox/tibs/pysource2/.

Version 2.0 of Ed Loper's Epydoc
supports reStructuredText-format docstrings for HTML output. Docutils
0.3 or newer is required. Development of a Docutils-specific
auto-documentation tool will continue. Epydoc works by importing
Python modules to be documented, whereas the Docutils-specific tool,
described above, will parse modules without importing them (as with
HappyDoc, which doesn't support
reStructuredText).

The advantages of parsing over importing are security and flexibility;
the disadvantage is complexity/difficulty.

Security: untrusted code that shouldn't be executed can be parsed;
importing a module executes its top-level code.

Flexibility: comments and unofficial docstrings (those not supported
by Python syntax) can only be processed by parsing.

Complexity/difficulty: it's a lot harder to parse and analyze a
module than it is to import and analyze one.